As suspected in a previous article from HSC the Panasonic G9 bitrate for VFR or high frame rate mode is much lower than on the GH5. The Manual has been released for the G9 showing this lower spec. It is of note that the codec used on both cameras is very efficient which lends itself to good quality even at 20mbps. However, for those that want to prioritize fine detail and the utmost sharpness, the GH5 will, in the end, be the better option for slow motion.
There is also the upcoming announcement rumor of the GH5s Lumix camera now with leaked images here with 240fps at full HD using a completely new sensor which may be able to improve of the GH5 slow-motion quality which in our analysis was lacking when using 150fps and 180fps on the regular GH5. Aliasing and moire were evident in footage which is not present in the 120fps mode. Considering the GH5s might be recycling the same image processing engine but with a new sensor, we may still see some artifacts.